feat: add incoming ref data to non-flakes entrypoint

We can now implement a Nix library for Buildbot CI. :)

We dump it into a file, it's better to pass large stuff and easier to
escape things.

Signed-off-by: Raito Bezarius <raito@lix.systems>

buildbot_nix/__init__.py

@@ -8,6 +8,7 @@ from collections.abc import Generator
 from dataclasses import dataclass, field
 from pathlib import Path
 from typing import TYPE_CHECKING, Any
+import tempfile
 
 import buildbot
 from buildbot.configurators import ConfiguratorBase
@@ -453,13 +454,14 @@ class NixEvalCommand(buildstep.ShellMixin, steps.BuildStep):
 
         return result
 
-def make_job_evaluator(name: str, settings: EvaluatorSettings, flake: bool) -> NixEvalCommand:
+def make_job_evaluator(name: str, settings: EvaluatorSettings, flake: bool, incoming_ref_filename: str) -> NixEvalCommand:
     actual_command = []
 
     if flake:
         actual_command += ["--flake", f".#{FLAKE_TARGET_ATTRIBUTE_FOR_JOBS}"]
     else:
-        actual_command += ["--expr", "import ./.ci/buildbot.nix"]
+        actual_command += ["--expr",
+                           f"import ./.ci/buildbot.nix {{ incoming_ref_data = builtins.fromJSON (builtins.readFile {incoming_ref_filename}); }}"]
 
     return NixEvalCommand(
         env={},
@@ -514,7 +516,8 @@ class NixConfigure(buildstep.CommandMixin, steps.BuildStep):
                     make_job_evaluator(
                         "evaluate `.ci/buildbot.nix` jobs",
                         self.evaluator_settings,
-                        False
+                        False,
+                        "incoming-ref.json"
                     )
                 ]
             )
@@ -527,7 +530,8 @@ class NixConfigure(buildstep.CommandMixin, steps.BuildStep):
                     make_job_evaluator(
                         "evaluate `flake.nix` jobs",
                         self.evaluator_settings,
-                        True
+                        True,
+                        "incoming-ref.json"
                     )
                 ]
             )
@@ -616,6 +620,11 @@ def nix_eval_config(
         lock=eval_lock
     )
 
+    # This information can be passed at job evaluation time
+    # to skip some jobs, e.g. expensive jobs, etc.
+    # Transfer incoming ref data to the target.
+    factory.addStep(steps.JSONPropertiesDownload(workerdest="incoming-ref.json"))
+
     # NixConfigure will choose
     # how to add a NixEvalCommand job
     # based on whether there's a flake.nix or